Base stations in modern and next generation cellular communications networks transmit wideband signals. As such, a Power Amplifier (PA) of such as base station must be able to achieve wide, or broad, bandwidths that cover more than 10% relative frequency bandwidth (i.e., greater than 10% bandwidth relative to a center frequency of operation). Notably, relative frequency bandwidth is defined as Δf/fo, where Δf=(fH−fL), fo=[(fH+fL)/2], and fH and fL are an upper frequency and a lower frequency that define the outer edges of the bandwidth of operation. The most common high efficiency power amplifier architecture that is currently being implemented is the Doherty amplifier. As illustrated in FIG. 1, a conventional Doherty amplifier 10 includes a splitter 12, a class AB main amplifier 14, a class C or class B peaking amplifier 16, and a combining network 18 (sometimes referred to as a “Doherty combining network”) connected as shown. An efficiency enhancement of the Doherty amplifier 10 is implemented through load modulation of the main amplifier 14 from the peaking amplifier 16 at peak and back-off power levels. This load modulation is implemented through the combining network 18, which typically consists of two λ/4 (quarter wavelength) impedance transformers 20 and 22 with a high transformation ratio connected between the main and peaking amplifiers 14 and 16 and an external load 24 as shown. Due to the inherent band limiting characteristics of the λ/4 impedance transformers 20 and 22, the Doherty amplifier 10 tends to only support a narrow bandwidth (1-5% relative bandwidth) that is catered for a specific band of operation.
There are several existing approaches to design wider band impedance transformers that can be used in the combining network 18 of the Doherty amplifier 10. For instance, as taught in U.S. Pat. No. 7,602,241 and illustrated in FIG. 2 (which corresponds to FIG. 3B of U.S. Pat. No. 7,602,241), the bandwidth limitation of a single λ/4 impedance transformer can be overcome by using cascaded-connected impedance transformers that perform the same impedance conversion at each of N frequencies. However, the improvement in bandwidth carries a cost of substantial physical size (i.e., the cascade-connected impedance transformer is 2 to 5 times longer than a λ/4 impedance transformer). This increase in physical size is not compatible with current space limitations on Printed Circuit Boards (PCBs) in modern base stations.
In a similar manner, as taught in U.S. Pat. Nos. 8,193,857 and 8,339,201 and illustrated in FIG. 3 (which corresponds to FIG. 1 of U.S. Pat. No. 8,193,857), a gradual tapered impedance transformer 22″, such as a Klopfenstein impedance transformer, rather than an impedance transformer having multiple impedance steps can be used to achieve wider bandwidth. However, the taper of the impedance transformer 22″ must be gradual and, as a result, the physical length of the impedance transformer 22″ would be considerably longer than a single λ/4 impedance transformer for a good tapered line. Therefore, again, the improvement in bandwidth carries a cost of substantial physical size. Also, due to the nature of the tapered structures, the size of the impedance transformer 22″ cannot be minimized by using common techniques such as bending.
To achieve multi-band performance, lumped element solutions have also been suggested as illustrated in FIGS. 4 and 5, where capacitors, circulators, varactors/diodes, hybrids, and controllers are implemented as part of the impedance transformation process to achieve wider bandwidth. Specifically, as taught in U.S. Pat. No. 8,314,654 and illustrated in FIG. 4 (which corresponds to FIG. 1 of U.S. Pat. No. 8,314,654), the λ/4 impedance transformer 20 can be replaced by a tunable impedance inverter 26 that uses capacitors as a tuner and a circulator and is controlled by a digital controller 28. FIG. 5, which corresponds to FIG. 2 of U.S. Pat. No. 8,314,654, shows another implementation where the splitter 12 is implemented as a hybrid coupler, the tunable impedance inverter 26 is implemented as a hybrid coupler, and offset line circuits 30 and 32 are used to couple the main and peaking amplifiers 14 and 16 to the tunable impedance inverter 26 and the λ/4 impedance transformer 22, as illustrated. One problem with the lumped element solutions proposed in U.S. Pat. No. 8,314,654 is that these solutions require a number of components and an elaborate system to implement, which would result in a costly and complex solution.
As such, there is a need for a wideband impedance transformer for a combining network of a Doherty power amplifier that is physically small and has low-complexity.